1. Field of the Invention
The present invention relates to imaging microscopes and, more particularly, to FT-IR (Fourier transform--infrared) imaging microscopes.
2. Description of the Related Art
When imaging a sample using a FTIR scanning microscope, it is commonplace to examine an image of the sample as an array of spatial regions. This division of the sampling region into individual elements, or "pixels", allows the analog signal for each pixel to be individually detected and converted to a digital signal. The entire area of the sample image is thereby digitized to produce a digital image which is more conducive to post-processing techniques. A conventional microscope accomplishes this breakdown of the sample into pixels with a raster-by-raster scan, examining each pixel region of the sample one at a time. For obvious reasons, this is a particularly lengthy process.
More recently, prior art systems have been making use of a multi-pixel detector array. The multi-pixel array has a grid of detection elements, each of which detects a different spatial region of the light passing through the sample. This allows for independent detection of different regions of an image. This removes the need to examine the pixels one at a time. However, in practice, the multi-pixel array suffers from a number of drawbacks.
For obtaining images of good quality resolution, it is necessary to use a sufficient number of pixels. For example, a typical imaging arrangement might use an array having pixel dimensions of 100.times.100, resulting in 10,000 individual pixels. Because of the complexity of these devices, they tend to be very expensive. For the same reason, they are also difficult to interface with other detection equipment. In addition, they tend to have a limited range of detection wavelengths, roughly 1000 cm.sup.-1 -3000 cm.sup.-1, whereas a the normal response in imaging experiments is 400 cm.sup.-1 -8000 cm.sup.-1.
Perhaps the most notable disadvantage of multi-pixel detector arrays is the need to convert the analog signal of each pixel to a corresponding digital signal. Analog-to-digital (A/D) converters are generally limited to no more than 20 channels. Thus, for a detector having 10,000 pixels, 500 twenty-channel converters are required. Furthermore, each pixel is typically scanned a number (e.g. 100) of times, and the multiple scans are averaged together prior to digitizing so as to improve the signal-to-noise ratio. Thus, there is a significant amount of time required for the scanning of each pixel, and the overall experiment can last many hours.
A recent method of producing a digitized scan avoids many of the difficulties associated with the multi-pixel detector. This method is described in U.S. Pat. No. 4,615,619 to Fateley, and relies on a single-element detector, rather than a multi-pixel array. The division of the image into pixels is accomplished by an electrically-controllable mask. The mask uses a liquid crystal material arranged in pixel zones on the surface of the mask, the liquid crystal material being relatively transmissive when free of applied electrical current, but relatively opaque when a current is applied. In the mask, each zone is individually controllable to allow the liquid crystal material of a particular zone to be made transmissive or opaque as desired. The lack of complete opacity in the liquid crystal results in imperfect masking of the source radiation. However, the method of Fatelely device nonetheless enables pixel regions of the sample to be defined.
The Fateley method uses the mask to define the desired shape of the scanning area, and the size and shape of the pixels being used. The sample material is then illuminated with the desired IR radiation, and the resulting spectroscopic image of the sampled collected by the single element detector. Typically, a plurality of images are collected, representing the desired number of scans. These scans are subsequently averaged together to produce a single spectroscopic image of the sample which has a better signal-to-noise ratio than any single scan.
To resolve the signal components for the different pixels defined by the mask, the signal detected by the single element detector is processed with a Hadamard transform. The Hadamard transform is calculated using limits defined by the pixel locations on the mask, and effectively performs a spatial demultiplexing of the signal data of the individual pixels. The individual signal components obtained from the transform are thus representative, respectively, of the spectral composition of each separate region of the sample defined by the mask.